1 files changed, 135 insertions, 0 deletions
diff --git a/internal/paging/boundary.go b/internal/paging/boundary.go
new file mode 100644
index 000000000..2f202097b
--- /dev/null
+++ b/internal/paging/boundary.go
@@ -0,0 +1,135 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package paging
+
+// MinID returns an ID boundary with given min ID value,
+// using either the `since_id`,"DESC" name,ordering or
+// `min_id`,"ASC" name,ordering depending on which is set.
+func MinID(minID, sinceID string) Boundary {
+	/*
+
+	           Paging with `since_id` vs `min_id`:
+
+	                limit = 4       limit = 4
+	               +----------+    +----------+
+	     max_id--> |xxxxxxxxxx|    |          | <-- max_id
+	               +----------+    +----------+
+	               |xxxxxxxxxx|    |          |
+	               +----------+    +----------+
+	               |xxxxxxxxxx|    |          |
+	               +----------+    +----------+
+	               |xxxxxxxxxx|    |xxxxxxxxxx|
+	               +----------+    +----------+
+	               |          |    |xxxxxxxxxx|
+	               +----------+    +----------+
+	               |          |    |xxxxxxxxxx|
+	               +----------+    +----------+
+	   since_id--> |          |    |xxxxxxxxxx| <-- min_id
+	               +----------+    +----------+
+	               |          |    |          |
+	               +----------+    +----------+
+
+	*/
+	switch {
+	case minID != "":
+		return Boundary{
+			Name:  "min_id",
+			Value: minID,
+			Order: OrderAscending,
+		}
+	default:
+		// default min is `since_id`
+		return Boundary{
+			Name:  "since_id",
+			Value: sinceID,
+			Order: OrderDescending,
+		}
+	}
+}
+
+// MaxID returns an ID boundary with given max
+// ID value, and the "max_id" query key set.
+func MaxID(maxID string) Boundary {
+	return Boundary{
+		Name:  "max_id",
+		Value: maxID,
+		Order: OrderDescending,
+	}
+}
+
+// MinShortcodeDomain returns a boundary with the given minimum emoji
+// shortcode@domain, and the "min_shortcode_domain" query key set.
+func MinShortcodeDomain(min string) Boundary {
+	return Boundary{
+		Name:  "min_shortcode_domain",
+		Value: min,
+		Order: OrderAscending,
+	}
+}
+
+// MaxShortcodeDomain returns a boundary with the given maximum emoji
+// shortcode@domain, and the "max_shortcode_domain" query key set.
+func MaxShortcodeDomain(max string) Boundary {
+	return Boundary{
+		Name:  "max_shortcode_domain",
+		Value: max,
+		Order: OrderDescending,
+	}
+}
+
+// Boundary represents the upper or lower limit in a page slice.
+type Boundary struct {
+	Name  string // i.e. query key
+	Value string
+	Order Order // NOTE: see Order type for explanation
+}
+
+// new creates a new Boundary with the same ordering and name
+// as the original (receiving), but with the new provided value.
+func (b Boundary) new(value string) Boundary {
+	return Boundary{
+		Name:  b.Name,
+		Value: value,
+		Order: b.Order,
+	}
+}
+
+// Find finds the boundary's set value in input slice, or returns -1.
+func (b Boundary) Find(in []string) int {
+	if zero(b.Value) {
+		return -1
+	}
+	for i := range in {
+		if in[i] == b.Value {
+			return i
+		}
+	}
+	return -1
+}
+
+// Query returns this boundary as assembled query key=value pair.
+func (b Boundary) Query() string {
+	switch {
+	case zero(b.Value):
+		return ""
+	case b.Name == "":
+		panic("value without boundary name")
+	default:
+		return b.Name + "=" + b.Value
+	}
+}